This invention relates to thermally activated electrochemical cells and more particularly to thermal activated electrochemical cells including a novel electrolyte system.
Thermally activated electrochemical cells or batteries have been used quite extensively in military applications, such as a power source for arming electronic ordnance fuses, and in fire detection devices because of their long shelf life and compactness. Batteries of this type typically include an electrolyte which, under normal storage conditions, is solid and does not conduct electricity. When the battery and/or the electrolyte is heated to a predetermined temperature, the electrolyte, upon changing to a molten or liquid state, becomes electrically conductive and cooperates with electrodes to provide the desired electromotive force.
The most widely used electrolytes for such batteries are inorganic salts or eutectic salt mixtures which typically melt at a temperature range of about 350 to 500.degree. C. Because of their relatively high latent heat of fusion, these salts require a substantial period of heating before becoming molten or liquid. Also, in order to insure that these electrolytes remain in a molten or liquid state for the desired time interval, it is often necessary to heat them above their fusion or solution temperatures. Consequently, batteries using these electrolytes are not particularly adaptable for use in fire detection devices and, when used as the power source for arming ordnance devices, must be provided with substantial insulation to prevent loss of heat which can cause the salt to refreeze with a resultant premature deactivation of the battery.
Various mixtures and complexes of inorganic salts have been proposed for use as electrolytes in thermally activated batteries. Representative examples of such electrolytes are disclosed in U.S. Pat. Nos. 3,819,415 (Benderly et al), 3,811,948 (Bald), 3,764,385 (Langer et al) and 3,117,032 (Panzer). While eliminating many of the problems associated with inorganic salts or eutectic salt mixtures, these electrolytes still have certain shortcomings such as poor conductivity, excessively high melting points for many applications, melting points which vary over relatively wide ranges, etc.